Gotcha.
I ask because his Sefer is a likkut - a gathering of information from various seforim, and much of it is sourced from late acharonim, and not all the sources he quotes are necessarily universally 'ongennumen'.
I am a huge fan of the Sefer, after seeing it lying around and browsing through it on several occasions I made a special request for my local seforim dealer to get it in.
It's well referenced - crucial for a good likkut (both in terms of deeming it reliable, and of allowing follow up analysis of sources), and he has relatively comprehensive footnotes on the page. IMHO these two characteristics often make the difference between a nice book, and an effective and (at least) somewhat complete piece of work.
However, a significant amount of what he writes in the 'pnim' comes from late acharonim that are not necessarily universally accepted as 'primary poskim' which is why I asked for a reference, as Id be interested to see the details behind the statement.
AFAIU, timtum haleiv is generally associated with the fact that the issur that is ingested is subsequently digested and absorbed into the bloodstream, and molecules obtained from the food are then incorporated into the body of the individual - "naaseh dam u'bossor mi'bsoro", resulting in a body which is partly built out of issur, and therefore has a reduced sensitivity to Kedusha. It would presumably follow, that regarding removal of issur from an individual's digestive system,, 'the sooner the better', and the longer it stays, the more of it will be absorbed into the bloodstream and then incorporated into the body, and the greater the degree of timtum haleiv likely to result. It seems doubtful to me that it's a matter of seconds before complete timtum haleiv has been achieved, and I would relate to a proposal that it's a significant period of time before food has completely moved on from the stomach to the duodenum (first part of the small intestine) and is unable to be removed from the system by regurgitation, which would prevent it from being absorbed.
Regarding the time after eating that one can still make a brocho achrono, AFAIK the practical gauge is 'as long as one still feels some of the satisfaction resulting from the consumption of the food' which is a halachic indication that some of it still remains in the stomach. I write this offhand, I'm not currently next to a sefer that I can confirm and quote, but if this is indeed the case, it would indicate that food can remain in the stomach for significant periods of time after consumption - depending on the variety and quantity of food consumed, and if the cheshbon about absorption and incorporation of issur into the body causing timtum haleiv is accurate, it may indicate a to'eles in removing issur from the system as long as it is still possible.
There are gaps as not all of what i wrote is referenced to a Torah source at the moment, but it's what is on my mind offhand.